In general, an ultrasonic transducer includes a pair of electrodes sandwiching a layer including a piezoelectric material (hereinafter referred to as a piezoelectric layer), and an ultrasonic probe includes a plurality of the ultrasonic transducers, where the ultrasonic transducers are one-dimensionally arrayed, for example. Further, a predetermined number of transducers of the transducers arrayed in the major-axis direction are determined to be an aperture, the plurality of transducers belonging to the aperture is driven, and an ultrasonic beam converges to a part to be measured in a patient so that the part is irradiated with the ultrasonic beam. Further, the plurality of transducers belonging to the aperture receives an ultrasonic reflective echo or the like emitted from the patient and the ultrasonic reflective echo is converted to an electrical signal.
On the other hand, as for the minor-axis direction perpendicular to the above-described major-axis direction, an aperture-width is modified by changing the frequency of an ultrasonic wave so that the beam-width of the ultrasonic beam decreases and the resolution increases (Patent Document 1: JP7-107595A). In an ultrasonic probe according to Patent Document 1, the thickness of a piezoelectric layer at the center in the minor-axis direction is small and gradually increases toward the end thereof. Therefore, the response to a high frequency at the center is high and the response to a low frequency at the end in the minor-axis direction is high, so that a wide-band frequency characteristic is obtained. As a result, the aperture-width in the minor-axis direction of the ultrasonic probe varies inversely with a frequency, whereby a fine beam-width is achieved over an area ranging from a shallow depth to a deep depth.
However, according to the ultrasonic probe disclosed in Patent Document 1, the low-frequency responses at both ends in the minor-axis direction become higher than that at the center part and the sound pressure at each of the ends is higher than that at the center part, whereby a nonuniform sound-pressure distribution is obtained. Subsequently, the resolution of the ultrasonic probe decreases.